The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Powertrain systems incorporate a plurality of torque-generative devices attached to an electrically variable transmission device for use in vehicle propulsion systems. The torque-generative devices typically include internal combustion engines and electrical machines powered using electrical energy storage devices, e.g., high voltage batteries. Torque outputs of the torque-generative devices are combined in the transmission device to transmit tractive torque to a driveline of the vehicle, and to transmit torque between the various devices and the driveline.
A typical electrically-variable transmission (EVT) splits mechanical power that is transmitted from an input shaft to an output shaft into a mechanical power path and an electrical power path by means of differential gearing. The mechanical power path typically includes gears and may also include clutches. The electrical power path employs electrical power machines, each of which may operate as a torque-generative motor or as an electrical-generative generator. Varying the speed of at least one of these electrical machines varies the transmission speed ratio, i.e., the ratio of input shaft speed to output shaft speed. The EVT can be incorporated into a propulsion system for a hybrid electric vehicle using an electrical energy storage device. A typical hybrid propulsion system uses electrical energy storage devices and an internal combustion engine as power sources. The batteries are connected with the electrical drive units through an electronic control system which manages and distributes power as required. The control system also has connections with the engine and vehicle to determine operating characteristics, or operating demand, permitting operation of the electrical power units as either motors (generating tractive torque) or a generator (generating electrical energy). When operating as a generator, the electrical power unit accepts power from either the vehicle or the engine and stores power in the battery, or provides that power to operate another electrical device or another electrical power unit on the vehicle or on the transmission.
A benefit of having an electrically-variable transmission incorporating more than one mode of operation is that each mode of operation will generally incorporate at least one mechanical point where one of the electrical power units is stationary, thereby reducing the electrical power input and providing a pure mechanical power flow path to improve system efficiency. Operating in more than one mode has required two or more clutches to change the mechanical configuration among the input, output and a single pair of electrical machines. Since an objective of the electrically-variable transmission is overall efficiency, it is advantageous to provide multiple mechanical points at which one of the electrical power units is stationary while minimizing the addition of components which would produce additional mechanical losses, i.e., spin losses, such as clutches, pumps and so on. Thus, an improved electrically variable transmission is now described.